1. Field of the Invention
The present invention relates to a light-emitting diode driver circuit and a lighting apparatus.
2. Description of the Related Art
A certain type of a lighting apparatus employing a light-emitting diode (hereinafter, referred to as “LED”) is turned on with a power voltage from a commercial power supply. Generally, in such a lighting apparatus, a DC voltage for driving the LED is generated out of an AC voltage from the commercial power supply, using an AC-DC converter (see Japanese Patent Application Laid-Open Publication No. 2009-134945). FIG. 8 depicts a common configuration of an AC-DC converter. An AC-DC converter 100 is a circuit that generates a desired DC output voltage Vout out of an AC voltage Vac from a commercial power supply and drives an LED 300. The AC-DC converter 100 includes a full-wave rectifier circuit 200, capacitors 201 to 203, a resistor 204, a control circuit 205, a power MOSFET 206, diodes 207 and 208, a transformer 209, and a voltage detecting circuit 210.
When the AC-DC converter 100 is supplied with the AC voltage Vac, the full-wave rectifier circuit 200 full-wave rectifies the input AC voltage Vac to and outputs the rectified voltage Vac. The capacitor 201 smoothes a voltage output from the full-wave rectifier circuit 200 into an input voltage Vin. The capacitor 202 is charged with the smoothed input voltage Vin via the resistor 204 for starting the control circuit 205. The control circuit 205 uses a charging voltage of the capacitor 202 as a source voltage. Thus, the control circuit 205 starts up when the capacitor 202 is charged, and starts switching control over the power MOSFET 206. When switching control over the power MOSFET 206 is started, a voltage is generated across a primary coil L1 of the transformer 209, and as a result in response to a voltage change across the primary coil L1, a voltage is generated across each of a secondary coil L2 and an auxiliary coil L3 of the transformer 209. A current generated by the auxiliary coil L3 of the transformer 209 is rectified by the diode 207, to be supplied to the capacitor 202. Therefore, after the start of the control circuit 205, the source voltage of the control circuit 205 is secured in a stable manner with a voltage from the auxiliary coil L3 of the transformer 209 through the diode 207.
The diode 208 and the capacitor 203 rectify and smooth a voltage from the secondary coil L2 of the transformer 209. Thus, a DC charging voltage is generated across the capacitor 203. The voltage detecting circuit 210 compares the output voltage Vout, which is the charging voltage of the capacitor 203, with a desired voltage. When the output voltage Vout is higher than the desired voltage, the voltage detecting circuit 210 allows the control circuit 205 to extend a time period during which the power MOSFET 206 is off. On the other hand, when the output voltage Vout is lower than the desired voltage, the voltage detecting circuit 210 allows the control circuit 205 to extend a time period during which the power MOSFET 206 is on.
Therefore, in the AC-DC converter 100, the output voltage Vout becomes the desired voltage, and the desired voltage is applied to the LED 300.
The AC voltage Vac has a frequency of 50 Hz, for example, and thus an electrolytic capacitor having a large capacitance is used as the capacitor 201 which smoothes a full-wave rectified voltage. In the AC-DC converter 100, even if a current, etc., passing through the LED 300 transitionally vary, an electrolytic capacitor having a large capacitance is also used as the capacitor 203 so that the fluctuation in the output voltage Vout is suppressed. As such, an electrolytic capacitor having a life shorter than that of a ceramic capacitor, etc., is used in the AC-DC converter 100, which causes such a problem that maintaining the life of the AC-DC converter 100 longer than that of the electrolytic capacitor is difficult.